The goal of our studies is to identify and analyze the neural elements of spinal pain and temperature pathways in the primate. Our experiments are designed to compare the three major areas oF the spinal grey which receive nociceptive afferents and contain nociceptive spinothalamic tract (STT) and spinoreticular (SR) projection neurons. These areas are laminae I and II of the superficial dorsal horn, lamina V of the dorsal horn, and the area surrounding the central canal, lamina X. We are using three interrelated approaches to understand these areas; 1. Identification by immunocytochemistry of several different neurochemically defined elements of laminae I, V and X, with particular emphasis on those associated with small diameter afferents and descending or spinal neurons involved in modulation of nociceptive circuits. Their relationship with spinothalamic neurons is detected by retrograde filling of these cells with Peroxidase. This basic description of types of neurons and terminals in the 3 related areas is needed for the interpretation of the relationships observed in all our experiments. 2. Analysis of neurochemically identified synaptic terminals contacting spinothalamic neurons which have been physiologically recorded and then intracellularly filled with HRP. We are quantitatively analyzing the synaptic contacts onto the soma and dendrites, particularly those dendrites that extend into laminae I-II. V and X from a single neuron. These experiments can reveal much about the integration of synaptic inputs by the STT cells. The entire dendritic field is available for EH analysis, the cells are physiologically identified, several cells of one type (WDR) can be cross compared, and the cells extend their processes into each of the 3 spinal areas of interest. 3. Analysis of visceral and somatic afferents. Convergence onto spinothalamic and spinoreticular neurons. Primary afferents of a visceral nerve and a somatic nerve. as well as their target neurons, will be differentially labeled for ultrastructural study of their interrelationships. Little is known of visceral afferent ultrastructure, nor of the morphological organization of viscerosomatic convergence onto specific Projection neurons.